1. Field of the Invention
The present invention relates to a transistor integrated circuit apparatus, and more specifically to a transistor integrated circuit apparatus usable for, for example, a power transistor circuit for amplifying an RF signal.
2. Description of the Background Art
For a transistor circuit for amplifying an RF signal, a circuit including a plurality of transistors (for example, hetero-junction bipolar transistors) connected in parallel is conventionally used in order to guarantee good RF characteristics.
For example, a circuit shown in FIG. 4 is conventionally used as a power transistor circuit. The power transistor shown in FIG. 4 includes n number of transistors Tr41 through Tr4n, a bias resistor R41, a capacitor C41, a DC power supply input terminal DCin, a RF signal input terminal RFin, and a RF signal output terminal RFout.
In the power transistor circuit shown in FIG. 4, a DC voltage (bias voltage) is applied to a base electrode of each of the transistors Tr41 through Tr4n via the common bias resistor R41. An RF signal is also input to the base electrode of each of the transistors Tr41 through Tr4n from the RF signal input terminal RFin via the capacitor C41. An emitter electrode of each of the transistors Tr41 through Tr4n is grounded. A collector electrode of each of the transistors Tr41 through Tr4n is commonly connected to the RF signal output terminal RFout. An output signal from each of the transistors Tr41 through Tr4n is output to the outside of the circuit via the RF signal output terminal RFout.
The power transistor circuit shown in FIG. 4 has an ideal circuit configuration which is designed assuming that there is no variance in the characteristics among the transistors. Since there are, however, variances in the characteristics among the transistors in actuality, variances occur in the operation of the transistors. Therefore, in the power transistor circuit shown in FIG. 4, a part of the transistors may cause thermal runaway due to an excessive amount of heat generated during the operation. When a transistor causes thermal runaway, the transistor is deteriorated or damaged and this causes malfunction of the circuit.
In order to solve the above-described problem, a power transistor circuit shown in FIG. 5 is conventionally used. The power transistor circuit shown in FIG. 5 includes n number of transistors Tr51 through Tr5n, n number of bias resistors R51 through R5n, a DC power supply input terminal DCin, a RF signal input terminal RFin, and a RF signal output terminal RFout.
In the power transistor circuit shown in FIG. 5, the bias resistors R51 through R5n are respectively connected to base electrodes of the transistors Tr51 through Tr5n. Owing to such a configuration, an increase in the base current in each transistor is suppressed even when the temperature of the transistor is excessively raised. This prevents thermal runaway of the transistors Tr51 through Tr5n. However, in the circuit shown in FIG. 5, an RF signal flows through the bias resistors R51 through R5n, which lowers the gain. For this reason, the circuit configuration shown in FIG. 5 is not suitable for a power transistor circuit.
A method for dividing a power transistor circuit into cells is proposed in the specification of U.S. Pat. No. 5,608,353. FIG. 6 shows a circuit configuration of the power transistor circuit described in U.S. Pat. No. 5,608,353. The circuit shown in FIG. 6 includes n number of cells, each including a transistor circuit, connected in parallel. For example, a cell 600 includes a transistor Tr61, a capacitor C61, and a bias resistor R61.
A base electrode of the transistor Tr61 is connected to one end of the capacitor C61 and one end of the bias resistor R61. The other end of the capacitor C61 is connected to an RF signal input terminal RFin. The other end of the bias resistor R61 is connected to a DC power supply input terminal DCin. The Base electrode of the transistor Tr61 is supplied with a bias current from a DC power supply circuit (not shown) via the bias resistor R61. The base electrode of the transistor Tr61 also receives an RF signal which is input via the capacitor C61. The other cells have substantially the same configuration as the cell 600. In the following description, the elements included in the cell 600 will be used to describe the operation of the conventional power transistor circuit shown in FIG. 6.
In this power transistor circuit, the bias resistor R61 is connected to the transistor Tr61. Owing to such a configuration, when the temperature in the transistor Tr61 is excessively raised, an increase in the base current in the transistor Tr61 is suppressed. This prevents thermal runaway of the transistor Tr61, and thus prevents the transistor Tr61 from being deteriorated or damaged due to the thermal runway.
In this power transistor circuit, an RF signal is input to the transistor Tr61 without passing through the bias resistor R61. Therefore, the gain is not lowered by the bias resistor R61, unlike in the power transistor circuit shown in FIG. 5.
In the power transistor circuit shown in FIG. 6, a base electrode of the transistor Tr61 is connected to the DC power supply circuit via the bias resistor R61. The DC power supply circuit usually uses a transistor formed on a semiconductor substrate on which the power transistor circuit is provided.
The power transistor circuit shown in FIG. 6 has the following problem. The transistor included in the DC power supply circuit has a gain in a wide band ranging from the DC band to the RF band. Therefore, RF noise generated in the DC power supply circuit is input to the transistor Tr61, and is output from the transistor Tr61 in an amplified state. In addition, depending on the type of the bias resistor R61, thermal noise generated in the bias resistor R61 may be a problem.
Generally in a power transistor circuit for amplifying an RF signal, a large magnitude of current occasionally flows to the collector of the transistor. At this point, a large magnitude of current also flows to the base of the transistor for an instant. In the power transistor circuit shown in FIG. 6, when a large magnitude of current flows to the base, the bias voltage is lowered by the bias resistor, which lowers the base potential. When the base potential is lowered, the operation state of the power transistor circuit becomes close to the class B operation. As a result, harmonics are generated in the collector current, and thus the peak voltage of the collector current is increased for an instant. This deteriorates the RF characteristics of the transistor; and in the worst case, damages the transistor.